Hi Chris, the main reason for not cupping the grip is because it allows the shooter to put un-even upwards pressure on the mag... Most will pop along without issue, but every once and awhile, the toggle won't lock down in the very last little bit of travel?? Not a big problem, but disconcerting when the gun was previously 100% reliable??...
... I had this very issue this morning, only I had the butt down hard on a sandbag... changed my hold... issues went away!
... best to all, til...lat'r....GT